The invention relates to a process for making polyolefins. In particular, the invention is a high-temperature solution process for making olefin polymers, especially polyethylenes. The process is catalyzed by an indenoindolyl transition metal complex.
High-temperature solution processes for olefin polymerization require a thermally robust catalyst. A few Ziegler-Natta catalysts based on titanium and vanadium components meet this test; some can withstand reactor temperatures as high as 250xc2x0 C. or more. In a typical solution process, the temperature and pressure in the reactor exceed the critical points of the olefin reactants and any added solvent, and the reactor temperature exceeds the polymer melting point. Consequently, the reactor contents are always liquified or xe2x80x9cin solution.xe2x80x9d Typical reactor pressures are 2000 to 5000 psig when a solvent is used. The polymerization can be performed in one or more stages. When polymerization is complete, the molten polymer must be treated to remove catalyst residues, which can cause color problems if not removed. Volatiles are stripped, and the product is cooled, pelletized, and dried.
Another type of solution process uses a stirred-zone reactor and no added solvent. In this process, the liquified olefin reactant, e.g., ethylene, functions as a solvent. These processes operate at relatively high temperatures (200 to 300xc2x0 C.) and pressures (15,000 to 50,000 psig).
Solution processes are characterized by short residence times (normally less than 10 minutes, and often as little as a minute or two). Consequently, in addition to having temperature stability, the catalyst systems used in these processes must activate quickly and thoroughly. This contrasts sharply with the requirements for catalysts used in slurry and gas-phase processes, where residence times are longer and catalyst lifetime is more important. Thus, a catalyst that is valuable for slurry and gas-phase processes might be a poor choice for use in a high-temperature solution process, and vice-versa.
As noted earlier, the Ziegler-Natta catalysts commonly used in high-temperature solution polymerizations have some limitations, one of which is the need to remove catalyst residues to avoid color problems. A second limitation relates to polymer density. Because the Ziegler-Natta catalysts incorporate comonomers only to a degree under these conditions, it can be difficult to drive polymer densities much below about 0.93 g/cm3. Consequently, it is challenging to make LLDPE, a product that is valuable for packaging film and other products, by using the high-temperature solution process with commonly available catalysts.
xe2x80x9cSingle-sitexe2x80x9d catalysts (including metallocenes), the new generation of olefin polymerization catalysts, may offer benefits for high-temperature processes. Some varieties incorporate comonomers well, and the catalysts can often be left in the polymer without adversely affecting color or other properties. Unfortunately, many single-site catalysts are not stable above 100xc2x0 C., so activities increase with temperature only until the catalyst begins to degrade; then, activity dropsxe2x80x94often dramatically. In addition, many catalysts lose the ability to make high-molecular-weight polymer at higher temperatures. These drawbacks have been partially overcome by using xe2x80x9cbridgedxe2x80x9d ligands, which tend to make the organometallic complex more thermally robust. See, for example: Hasegawa et al., J. Polym. Sci., A 38 (2000) 4641; Yano et al., J. Mol. Catal. A 156 (2000) 133; and U.S. Pat. No. 6,207,774, wherein a borate-activated, diphenylmethylene-bridged zirconocene is used to make high-molecular-weight polymers at 150-200xc2x0 C. Excellent activity was observed for the zirconium complex in which the diphenylmethylene group bridges cyclopentadienyl and fluorenyl ligands.
Bridging, however, does not provide a completely satisfactory solution for high-temperature solution polymerizations, in part because the complex must ordinarily be used with an activator. Alumoxanes, the most well-known activators, have limited thermal stability above 160xc2x0 C., and their use in high-temperature processes has been discouraged. See, for example, U.S. Pat. No. 5,767,208, which teaches to use ionic borate activators rather than alumoxanes in a high-temperature solution process. Borate activators are also used in the high-temperature processes disclosed in U.S. Pat. Nos. 6,313,240, 6,291,609, and 6,207,774. The ""774 patent identifies another problem of alumoxanes: the large amount of expensive alumoxane needed to give reasonable activity with most metallocenes (see column 1, lines 40-46). The ""240 and ""609 patents teach to use hafnium as the transition metal to produce a higher molecular weight product compared with that available from using a similar zirconium complex.
Organometallic complexes that incorporate xe2x80x9cindenoindolylxe2x80x9d ligands are known. U.S. Pat. No. 6,232,260 teaches the use of indenoindolyl Group 3-10 metal complexes as catalysts for polymerizing olefins. The examples illustrate the use of a non-bridged bis(indenoindolyl)zirconium complex for making HDPE using a slurry process at 80xc2x0 C. to 110xc2x0 C. The ""260 patent generally teaches that comonomers can be used and that the indenoindolyl ligand can be bridged to another ligand. The reference indicates that, in addition to the indenoindolyl ligand, another xe2x80x9cpolymerization-stablexe2x80x9d ligand can be present. Cyclopentadienyl, indenyl, and fluorenyl are taught as equivalents for the polymerization-stable ligand (see column 3, lines 43-48). Versatility is an advantage of the complexes. By modifying the starting materials, a wide variety of indenoindolyl complexes can be prepared.
PCT Int. Appl. WO 99/24446 (Nifant""ev et al.) also teaches organometallic complexes that incorporate a Group 3-6 transition metal and an indenoindolyl ligand. In many of the complexes, the indenoindolyl group is bridged to another ligand, which is often a second indenoindolyl ligand. Nifant""ev uses the catalysts to make a variety of polyolefins, including HDPE, LLDPE, ethylene-propylene copolymers, and polypropylene. Nifant""ev teaches to use the catalysts with a high molar ratio of aluminum to transition metal, typically 1000-8000, for favorable activity. Nifant""ev uses unsupported complexes in a low-temperature process; all of the examples are performed at 30xc2x0 C. to 80xc2x0 C.
Neither Nifant""ev nor the ""260 patent teaches solution processes, and neither indicates a preference for a particular indenoindolyl complex for use in a high-temperature solution polymerization. The references are also silent about the desirability of using a particular indenoindolyl complex for a solution process with a short residence time.
In sum, the industry would benefit from additional single-site alternatives to the Ziegler-Natta catalysts currently used in high-temperature solution processes for olefin polymerization. However, single-site catalysts with the ability to make high-molecular-weight polymer, even at high temperature, are a must. Ideally, the single-site catalyst used in the process would activate quickly and thoroughly to meet the requirement of short residence time. A valuable process would take advantage of the inherent flexibility of the indenoindolyl ligand framework, and could utilize readily available alumoxane activators at low levels.
The invention is a high-temperature solution process for making polyolefins. Olefins polymerize rapidly (residence time  less than 10 minutes) in a reaction zone at a temperature greater than about 130xc2x0 C. in the presence of an activator and an organometallic complex. The complex includes a Group 3-10 transition metal and a bridged indenoindolyl ligand. One part of the ligand is a second indenoindolyl group or a polymerization-stable, cyclopentadienyl-like group having an extended pi-electron system. We found that complexes based on a particular kind of bridged indenoindolyl ligand are valuable for high-temperature solution polymerizations. Because the catalysts activate quickly and thoroughly, even with low levels of an alumoxane, the invention provides an efficient way to make high-molecular-weight (Mw greater than 100K) polyolefins.